Magnetic core with supporting and clamping structure



F. F. BRAND April 25, 1933.

MAGNETIC CORE WITH SUPPORTING AND CLAMPING STRUCTURE Filed Oct. 18 1952 6 rik:

T Inventor: Frederick P, Brand,

HisAttorneg.

Patented Apr. 25, 1933 UNITED STATES PATENT OFFICE FREDERICK F. BRAND, OF PITTSFIELD, MASSACHUSETTS, ASSIGNOR T GENERAL ELEG- TRIC COMPANY, A CORPORATION OF NEW YORK MAGNETIC CORE WITH SUPPORTING AND CLAMPING STRUCTURE Application med October 18, 1932. Seria1 No.638,311.

My invention relates to magnetic cores for transformers, reactors and similar electrical apparatus. Magnetic cores of this character are built up of layers or laminations of magnetic sheet material which are held in place by suitable metal supporting and clamping structures and the invention relates more particularly to these core supporting and clamping structures.

The general object of the invention is to provide a laminated magnetic core with an improved metal supporting and clamping structure so arranged as to reduce losses which commonly occur in such structures due 5 to the effect of magnetic leakage flux from the core. I

Another object of the invention is the provision of an improved core supporting and clamping structure so arranged as to avoid closed electrical circuits in the structure and thus a possibility of arcing or sparking with resultant decomposition of surrounding insulating liquid and the production of inflammable gas and danger of explosion.

A further object of the inventlon is to provide an improved metal supporting or clamping structure for a magnetic core having a plurality of winding legs, the supporting and clamping structure being arranged to permit the windings on any winding leg to be loaded independently of the other winding leg or legs without interference by the supporting and clamping structure with the leakage flux of any windings. t

A still further object of the invention is to provide a three phase or polyphase magnetic core with a metal supporting and clamping structure which will be neutral to the leakage magnetic field of zero sequence currents in the windings.

These and other advantages and objects will be apparent from the following description taken in connection with the accompanying drawing in which Fig. 1 is a perspective view of a simple, two-legged, single phase core type transformer provided with a core supporting and clamping structure in accordance with the invention; Fig. 2 is a partial perspective View of a modified form of the arrangement shown in Fig. 1; Fig. 3 is a erspective view of a simple, threeleggec, three phase core type transformer provided with a core supporting and clamping structure in accordance with the invention; Fig. at is'a partial perspective view of a modified form of the arrangement shown in Fig. 3; and Figs. 5 and 6 are sectional explanatory detail views.

Like reference characters indicate similar parts in the diiierent figures of the drawing.

The single phase transformer shown in Fig. 1 includes a two-legged magnetic core 10 with two winding legs surrounded by primary windings 11 and 12 and secondary windings 13 and 14 respectively. The core 10 is built up of superposed magnetic layers or laminations which are secured together by metal channel members or clamps 15 held firmly against the core yokes by metal connecting cross rods or ties 16. While the transformer is in normal operation, the currents in the primary and secondary windings produce leakage magnetic flux which follows the paths indicated by the broken lines 17 and 18, the relative instantaneous directions of the leakage flux in the two paths 17 and 18 being indicated by arrows.

It will be apparent from Fig. 1 that the two upper channel-shaped clamps 15 and their cross rods 16 form a closed loop around the upper end or yoke of the core 10 and in the field of the magnetic leakage flux from the two pairs of windings. It is also apparent that the leakage flux following the paths 17 and 18 passes through and interlinks the closed loop formed by the channels 15 and their tie rods 16. If the two winding legs of the core are exact duplicates and their windings are equally loaded, the magnetic leakage flux from the windings 11 and 13 on one winding leg and the magnetic flux from the windings 12 and 14 on the other winding leg produce equal and opposite voltages in the closed loop formed by the channels 15 and their cross rods 16 so that no current flows in this loop. However, it is not always possible nor even desirable that the two winding legs of a transformer be exact duplicates or that they or their windings be equally loaded. If the windings are not symmetrical and are not dupli cates or if the windings are not equally loaded, the voltages induced in the channels 15 and their cross rods 16 are not equal and opposite so that there is a resultant voltage tending to circulate a current in the closed loop formed by the channels 15 and their cross rods 16. This result is particularly apparent in the important class of transformers known as split-winding or double secondary transformers. In such transformers, the two secondary windings 13 and 14 may carry unequal loads. As an extreme case of unbalanced load, the secondary winding 13 may be idle while the secondary winding 14 is loaded, in which case there will be no leakage flux along the path 17 and the leakage flux along the path 18 will be unopposed and will induce voltage in the closed loop formed by the channels 15 and their tie rods 16, thus tending to produce a circulating current in this closed loop.

It has been previously recognized that a metal supporting and clamping structure of a transformer core must not form a short circuit around the path of the main core magnetizing flux and furthermore it must not short circuit the core laminations. Accordingly, it has been customary to provide insulation 19 between the cross rods 16 and the transformer core and to provide insulation 20 between the supporting and clamping channels 15 and the core. It has been customary, however, to secure the cross rods 16 to the channels 15 by screws or bolts 21 passing through bolt holes in the channels 15. The channels 15 and their cross rods 16 as previously arranged thus form an electrically closed loop, usually of moderately high resistance, around the leakage flux paths 17 and 18. The high resistance in this closed loop is usually due to the resistances of the contacts between the channels 15 and the adjacent ends of their cross rods 16. It is well known that a contact resistance varies with the character of the contact surfaces, with the pressure between the contact surfaces, and with the current flowing between the surfaces. Under these conditions the current tends to produce overheating and fusing or sparking at the contact, especially when the contact surfaces are not smooth and polished and the current density at the contact is too great. Thus such high resistance contacts when carrying electrical current may result in local heating and sparking and, when immersed in the usual insulating oil, the heat may be so great as to Carbonize and disintegrate the oil so as to reduce its dielectric strengdh and produce dangerous inflammable gases.

The above disadvantages are avoided by the present invention by insulating at leastsome of the joints between the cross rods 16 and the channels 15 as indicated in Fig. As shown in this figure, the insulation be tween a channel 15 and a tie rod 16 includes an insulating bushing 22 surrounding the bolt or screw 21, an insulating washer 25 between the channel 15 and the adjacent. end of the tie rod 16, and a second insulating washer 26 between the channel 15 and the head of the bolt or screw 21. With this arrangement, the two channels 15 with their cross rods 16 form a mechanically closed loop which, however, is electrically completely open with suflicient insulation to withstand the maximum voltage which may be induced in the loop by any unbalanced loading of the transformer even when one of its secondary windings is short circuited.

A modified supporting and clamping arrangement is shown in Fig. 2. This is similar to the arrangement shown in Fig. 1 except that the channels 15 are connected by additional metal cross ties or supports 27. These cross ties or supports 27 are secured to the channels 15 by bolts 28 insulated from the channels 15, as shown in Fig. 6, by insulating bushings 29 and insulating washers 30 and 31. Unless these cross members 27 are electrical- 1y insulated from the channels 15' it will be apparent from Fig. 2 that the leakage flux from the windings 11 and 13 on one winding leg of the core will induce voltage and tend to cause currents to flow in the loops formed by the channels 15 and the cross rods 16 and cross ties 27 at the adjacent end of the clamping structure. Similarly, the magnet c leakage flux from the windings 12 and 14 on the other winding leg of the core will induce a voltage and tend to cause current to flow in the corresponding parts at the other end of the clamping structure.

The transformer shown in Fig. 3 includes a three phase magnetic core 10 having three winding legs surrounded by primary windings 32, 33 and 34 and secondary windings 35. 36 and 37 respectively. During the normal operation of the transformer, leakage flux is produced which follows the paths indicated by the broken lines 38, 39 and 40. If the leakage fluxes in these three paths are equal in magnitude and are 120 apart in phase. the voltages which they induce in the closed loop formed by the two upper channels 15 and their cross rods 16 will neutralize each other. However, they are not likely to be equal in magnitude even if they are 120 apart because of the dissymmetry of the three core legs with reference to both the main and the leakage magnetic fields. This will be apparent on a comparison of the relative position of e ther outer winding le and that of the central winding leg. It will be particularly true if the three winding legs of the transformer are not loaded in exactly the correct ratio to make the fluxes in the three flux paths 38, 39 and equal in magnitude. Furthermore, these three fluxes will not even tend to induce voltages which neutralize each other in the supporting and clamping structure even though the fluxes are equal in magnitude if they are not 120 apart, as for instance, when they are produced by Zero sequence currents in the windings, in which case the three fluxes will be in phase with each other and the voltages induced by them in the closed loop formed by the clamping structure will add to each other arithmet cally. These three voltages thus tend to produce a large circulating current in the loop with attendant overheating and possible fusing and sparking at the contacts between the members forming the loop. The clamping members 15 and 16 forming the loop are therefore insulated from each other by suitable insulating bushings 22 and washers 25 and 26 as indicated in F g. 5 and as.

already described in connection with the single phase transformer shown in Fig. 1.

A three phase transformer similar to that shown in Fig. 4 with the exception that additional metal cross ties 27 are prov ded between the channels 15, these cross ties being insulated from the channels 15 by an insulating bushing 29 and insulating washers 30 and 31, as shown in Fig. 6 and already described in connection with Fig. 2. It is apparent from F g. 4 that unless these cross ties 27 are insulated from the channels 15, even though the three fluxes following the paths 38, 39 and 40 are exactly equal in magnitude and exactly 120 out of phase, voltages will be induced in the closed loops formed by the channels 15 and their cross rods 16 and cross ties 27. Thus currents will tend to flow in these loops causing undesirable local heating and possible sparking. The channels 15, cross rods 16 and cross ties 27 are therefore insulated from each other by bush ngs 22 and washers 25 andv 26 to prevent flow of current in the loops.

The invention has been explained bv de-" scribing and illustrating several pre erred forms thereof but it will be apparent that changes may be made without departing cally closed loop surroundin a leakage flux path, and said loop also including at least one insulated joint to prevent flow of current in the loop.

2. The combination with a laminated magnetic core having a winding leg, of windings on said winding leg, and a metal supporting and clamping structure for said core, said supporting and clamping structure forming mechanically closed and electrically open loops in the field of the leakage flux from said windings.

3. The combination with a laminated magnetic core having a winding leg, of windings on said winding leg, and metal members forming a supporting and clamping structure for said core, said metal members forming mechanically closed loops in the field of the leakage flux from said windings, at least some of said metalmembers being insulated from each other to prevent flow of current in said loops.

4. The combination with a laminated magnetic core, of windings arranged on said core to produce an unbalanced magnetic leakage field, and a metal supporting and clamping structure for said core, said supporting and clamping structure including metal members in said unbalanced leakage field, and at least some of said metal members being insulated from each other to prevent flow of current therein.

5. The combination in a single phase transformer, of a magnetic core having two winding legs, windings on said winding legs arranged to divide the transformer load unequall on said two legs, and a supporting and c amping structure for said core, said supporting and clamping structure including metal members forming mechanically closed and electrically open loops in the path of the leakage magnetic flux from said winding legs.

6.5 The combination in a single phase transformer, ofa magnetic core having a plurality from said core, and insulation between at least some of said metal members to prevent i flow of current in said loops. 7. The combination in a threepha-se transformer, of amagnetic core having three winding legs, three phase windings on saidwinding legs, and a supporting. and clamping structure for said core including two metal clamping members and metal cross members connecting said clamping members, at least some of said metal members being insulated from each other to prevent flow of current therein.

8. The combination in a polyphase transformer, of a magnetic core, polyphase windhand.

FREDERICK F. BRAND. 

